The present invention relates generally to pressure vessels, and more particularly provides a uniquely contructed two-piece, axially sectioned toroidal pressure vessel used to store and supply high pressure air utilized in various pneumatic control systems.
Conventional pneumatic control systems employ as their motive force a supply of high pressure air contained in a storage vessel which is operatively connected to the various air-driven components of the system through a pressure reduction system that functions to flow a regulated quantity of substantially lower pressure air to the driven components. Depending upon the space and weight limitations of the system, a wide variety of pressure vessel configurations may be used.
Particularly in space-limited applications, the toroidal shape has proven to be a very desirable storage vessel configuration because it permits various system structure, such as wiring and mechanical linkage, to be routed through the toroid's central opening. Thus, for example, in applications where the system must fit within a cylindrical housing of a predetermined inner diameter, a toroidal storage vessel of essentially the same overall diameter may be coaxially disposed within the housing at any point along its length and still permit the unimpeded interconnection of components positioned at opposite ends of the vessel.
Despite the desirability of its shape in many applications, however, the toroidal pressure vessel has heretofore presented several very difficult manufacturing problems which have significantly limited its use in high pressure air supply applications. It is to these problems that the present invention is directed.
The conventional method of fabricating a toroidal pressure vessel is to provide a section of metal tubing of an appropriate length and wall thickness, bend the tube section around a mandrel and butt weld the opposite tube ends together. Unfortunately, this seemingly simple and straightforward manufacturing technique is replete with inherent disadvantages and intricacies.
For example, it is well known that the area of maximum wall stress in an internally pressurized toroidal body occurs around the annulus of its radially innermost wall section. Thus, to equalize the pressure-induced stress around its cross-sectional area the radially inner wall of the vessel must be significantly thicker than its radially outer wall, with an appropriate degree of thickness tapering between these two extremes. Such equalization of wall stress is desirable, of course, because for a given internal design pressure and storage volume it minimizes the weight and external volume of the vessel. In the tube-bending method of forming the toroid, however, this desirable minimization is, as a practical matter, nearly impossible. Although, as the tube is bent there is a natural thickening of the resulting radially inner wall section, and a thinning of the radially outer wall section, the resulting thickness ratio (which, among other things, is dependent upon the tube section length) is nearly always far from optimal.
This unavoidable deficiency may be partially overcome by the relatively expensive and time-consuming expedient of custom manufacturing a tubing section having an eccentric bore. This is typically accomplished by drilling an axially offset bore in a section of solid cylindrical metal bar stock. The thicker wall portion of the eccentric tubing is then positioned against the mandrel prior to the bending of the tube into the requisite circular shape. As might be imagined, both the drilling and bending steps must be carried out with extreme care and precision to achieve an acceptable approximation of the optimum vessel cross-section. Not only must these steps be carefully performed, but precise design allowances must be made for the unavoidable wall thickness changes which occur during the bending process. In short, what would initially appear to be a straightforward design procedure in many instances turns out to be a time-consuming trial and error process with a concomitantly high scrap rate.
Another problem associated with the conventional tubebending method is that it is simply not feasable in the case of small-diameter, high pressure toroidal storage vessels. As a specific example, for an internal design pressure of 10,000 psi the lower internal diameter limit for the toroid is approximately four inches. At and below this diameter limit, metals strong enough to withstand the design pressure are not malleable enough to withstand the bending. Additionally, at these small toroidal diameters it is extremely difficult to properly butt weld the facing tube ends because of the very limited work space within the toroid's central opening.
Finally, because of the unavoidable imprecision as to resulting wall thicknesses in the finished pressure vessel an unnecessarily high safety factor must be utilized to assure that the design pressure limitation may be safely maintained. This necessity, of course, adds weight, external volume and expense to the finished vessel. Additionally, it is often a design requirement that the vessel have a predetermined burst location. Because of the wall thickness imprecision in the tube-bending method, however, this design requirement has also been difficult to meet.
Accordingly, it is an object of the present invention to provide a toroidal pressure vessel, and associated manufacturing methods therefor, which eliminates or minimizes above-mentioned and other problems and disadvantages associated with conventional storage vessels of toroidal configuration.